Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers
Authors: Trisha Greenhalgh, Danielle J. Cane, Matthew Oliver
Journal: Annals of Internal Medicine (2023)

LettersJuly 2023Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care WorkersFREETrisha Greenhalgh, PhD, Danielle J. Cane, MSc, Matthew Oliver, MEngTrisha Greenhalgh, PhDUniversity of Oxford, Oxford, United KingdomSearch for more papers by this author, Danielle J. Cane, MScCommunity Access to Ventilation Information (CAVI), Toronto, Ontario, CanadaSearch for more papers by this author, Matthew Oliver, MEngAlberta Utilities Commission, Edmonton, Alberta, CanadaSearch for more papers by this authorAuthor, Article, and Disclosure Informationhttps://doi.org/10.7326/L23-0072 SectionsAboutVisual AbstractPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail TO THE EDITOR: We read Loeb and colleagues' article (1) with interest. Some members of the evidence-based medicine community seem to assume that a randomized controlled trial, however imperfectly and illogically designed, is necessarily superior to other forms of evidence. This is not the case. Showing causality requires multiple types of evidence, including probabilistic (such as from controlled experiments) and mechanistic (that is, evidence supporting—or if causality is to be disproved, evidence refuting—the purported mechanism of action).Masking is a complex intervention. Its mechanism of action should have been taken into account when designing randomized controlled trials. A predominantly airborne transmission route for SARS-CoV-2 has been well established by multiple studies of different kinds. The mechanism by which respirators (for example, N95s) work to filter out airborne particles is also well established (2). Ferris and associates' study in a U.K. hospital showed a dramatic reduction in health care worker infections after introduction of respirator-grade respiratory protection (3), and a meta-analysis of 8 studies (9164 participants) showed that respirator use was associated with significantly fewer viral infectious episodes for health care workers compared with use of surgical masks (4).The airborne nature of SARS-CoV-2 means that health care workers may be exposed to the virus even when not directly caring for infected patients simply by breathing contaminated air. A valid comparison of respirators and medical masks in a hospital environment would therefore require respiratory protection to be worn continuously from entering to leaving the building. Instead, Loeb and colleagues instructed participants to wear the masks or respirators only when dealing with patients who had known or suspected COVID-19—a design that presupposes an exclusively close-contact mode of transmission (that is, contact or droplet). A plausible explanation for their findings is that health care workers became infected from the air after removing their respirators or masks. A systematic review showed that continuous wearing of N95 respirators during a health care worker's shift was associated with significant risk reduction in viral respiratory infections (5).Loeb and colleagues' findings are heavily skewed by data from Egypt collected at the time of a local outbreak of the highly contagious Omicron variant. An alternative explanation for the high rates of infection in both groups of the Egyptian study is that many participants were infected at home or in the community rather than in the workplace. This would bias findings toward the null.We were surprised that the editorial and peer-review process did not pick up these basic design flaws, which could explain the negative findings.References1. Loeb M, Bartholomew A, Hashmi M, et al. Medical masks versus N95 respirators for preventing COVID-19 among health care workers. A randomized trial. Ann Intern Med. 2022;175:1629-1638. [PMID: 36442064] doi:10.7326/M22-1966 LinkGoogle Scholar2. Brochot C, Saidi MN, Bahloul A. How effective is the filtration of ‘KN95’ filtering facepiece respirators during the COVID-19 pandemic. Ann Work Expo Health. 2021;65:358-366. [PMID: 33125464] doi:10.1093/annweh/wxaa101 CrossrefMedlineGoogle Scholar3. Ferris M, Ferris R, Workman C, et al. Efficacy of FFP3 respirators for prevention of SARS-CoV-2 infection in healthcare workers. Elife. 2021;10. [PMID: 34783656] doi:10.7554/eLife.71131 CrossrefMedlineGoogle Scholar4. Collins AP, Service BC, Gupta S, et al. N95 respirator and surgical mask effectiveness against respiratory viral illnesses in the healthcare setting: a systematic review and meta-analysis. J Am Coll Emerg Physicians Open. 2021;2:e12582. [PMID: 34746923] doi:10.1002/emp2.12582 CrossrefMedlineGoogle Scholar5. Yin X, Wang X, Xu S, et al. Comparative efficacy of respiratory personal protective equipment against viral respiratory infectious diseases in healthcare workers: a network meta-analysis. Public Health. 2021;190:82-88. [PMID: 33360295] doi:10.1016/j.puhe.2020.11.004 CrossrefMedlineGoogle Scholar Comments0 CommentsSign In to Submit A Comment Author, Article, and Disclosure InformationAuthors: Trisha Greenhalgh, PhD; Danielle J. Cane, MSc; Matthew Oliver, MEngAffiliations: University of Oxford, Oxford, United KingdomCommunity Access to Ventilation Information (CAVI), Toronto, Ontario, CanadaAlberta Utilities Commission, Edmonton, Alberta, CanadaDisclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=L23-0072. PreviousarticleNextarticle Advertisement FiguresReferencesRelatedDetailsSee AlsoMedical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Mark Loeb , Amy Bartholomew , Madiha Hashmi , Wadea Tarhuni , Mohamed Hassany , Ilan Youngster , Ranjani Somayaji , Oscar Larios , Joseph Kim , Bayan Missaghi , Joseph V. Vayalumkal , Dominik Mertz , Zain Chagla , Maureen Cividino , Karim Ali , Sarah Mansour , Lana A. Castellucci , Charles Frenette , Leighanne Parkes , Mark Downing , Matthew Muller , Verne Glavin , Jennifer Newton , Ravi Hookoom , Jerome A. 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Ross Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Mark Loeb , Dominik Mertz , Zain Chagla , John Conly Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Lisa M. Brosseau Metrics Current IssueJuly 2023Volume 176, Issue 7KeywordsCOVID-19Health care providersInfectious diseasesPreventive medicine ePublished: 18 July 2023 Issue Published: July 2023 Copyright & PermissionsCopyright © 2023 by American College of Physicians. All Rights Reserved.PDF downloadLoading ...